Matter devices run without internet access (at least this is the whole point of the spec, some manufacturers have fewer features without using the cloud based app, but to be Matter certified it must run locally to some extent), so blocking the vlan should be okay with a lot of IoT devices.
Random dodgy streamer box does need internet access though, so I think at best having a vlan (probably one just for it sadly) that doesn't have access to the rest of your internal network would be the only realistic solution. Still won't help prevent it from using your connection as part of a botnet though. It's a hard problem.
Unfortunately users are very adverse to learning anything about how their devices work, so I don't have any idea what can be done about the problem.
Maybe we have to rely on the state going after sellers of such pre-compromised devices? I'd say hold the users somewhat liable, maybe a small fine, when they are part of a botnet, and wave them when it's a "legit brand" that gets compromised outside of the users control? Pressure would need to be done on "legit" consumer manufacturers to actually provide security updates to somewhat older devices and not abandon them the minute the latest model is released.